System response of the sinoatrial node during vagal stimulation

The SA node response to modulations in canine vagal tone was investigated by means of the heart rate variability power spectrum. A new algorithm that was developed for accurate power spectrum estimation of short R-R segments is described. The performance of the algorithm was assessed for ECG recordings obtained from a controlled experiment, in which a frequency modulated pulse train was applied to the vagal nerve after vagal transaction and blockade of the sympathetic system. The power spectrum calculated for 20-25 heartbeats showed conspicuous spectral peaks in accordance with the different modulating frequencies between 0.1 and 0.3 Hz. The presence of spurious peaks was negligible even when the analysed signal segment consisted of only 20-25 beats. These results imply that for a certain range of modulating frequencies the sinoatrial node responds linearly to fluctuations in the parasympathetic tone. System identification methods that include fitting a linear model to the heart rate variability signal and analysis of the residuals were used for confirming the hypothesis of linearity. For higher frequencies of the modulating signal, usually above 0.3 Hz, the system was found to deviate from linearity.